In the recovery of oil from oil-bearing reservoirs, it usually is possible to recover only minor portions of the original oil in place by the socalled primary recovery methods which utilize only the natural forces present in the reservoir. A variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean reservoirs. The most widely used supplemental recovery technique is waterflooding which involves the injection of water into the reservoir. As the water moves through the reservoir, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.
It has long been recognized that factors such as the interfacial tension between the injected water and the reservoir oil, the relative mobilities of the reservoir oil and injected-water, and the wettability characteristics of the rock surfaces within the reservoir are factors which influence the amount of oil recovered by waterflooding. It has been proposed to add surfactants to the flood water in order to lower the oil-water interfacial tension and/or to alter the wettability characteristics of the reservoir rock. Processes which involve the injection of aqueous surfactant solutions are commonly referred to as surfactant waterflooding or as low tension waterflooding, the latter term having reference to the mechanism involving the reduction of the oil-water interfacial tension. Also, it has been proposed to add viscosifiers such as polymeric thickening agents to all or part of the injected water in order to increase the viscosity thereof, thus decreasing the mobility ratio between the injected water and oil and improving the sweep efficiency of the waterflood.
A problem with stability and effectiveness arises when these surfactants and thickeners are used in environments characterized by temperatures in the range of about 70.degree. C. to about 120.degree. C. and above, high pressures (e.g., up to about 4000 psi), high concentrations of divalent metal ions such as calcium, magnesium, etc. (e.g., up to 3000 ppm or more and in some instances as high as 10,000 or 20,000 ppm), and high salinity (e.g., total dissolves salts (TDS) levels of up to about 200,000 ppm).
Many waterflooding applications have employed anionic surfactants. For example, a paper by W. R. Foster Entitled "A Low-Tension Waterflooding Process", Journal of Petroleum Technology, Vol. 25, Feb. 1973, pp. 205-210, describes a technique involving the injection of an aqueous solution of petroleum sulfonates within designated equivalent weight ranges and under controlled conditions of salinity. The petroleum sulfonate slug is followed by a thickened water slug which contains a viscosifier such as a water-soluble biopolymer. This thickened water slug is then followed by a driving fluid such as a field brine which is injected as necessary to carry the process to conclusion.
One problem encountered in waterflooding with certain of the anionic surfactants such as the petroleum sulfonates is the lack of stability of these surfactants in so-called "hard water" environments. These surfactants tend to precipitate from solution in the presence of relatively low concentrations of divalent metal ions such as calcium and magnesium ions. For example, divalent metal ion concentrations of about 50-100 ppm and above usually tend to cause precipitation of the petroleum sulfonates.
Nonionic surfactants, such as polyethoxylated alkyl phenols, polyethoxylated aliphatic alcohols, carboxylic esters, carboxylic amides, and polyoxyethylene fatty acid amides, have a somewhat higher tolerance of polyvalent ions such as calcium or magnesium than do the more commonly utilized anionic surfactants. While it is technically feasible to employ a nonionic surfactant solution to decrease the interfacial tension between the injected aqueous displacing medium and petroleum contained in some limestone formations, such use is generally not economically feasible for several reasons. Nonionic surfactants are not as effective on a per mole basis as are the more commonly used anionic surfactants and, additionally, the nonionic surfactants generally have a higher cost per unit weight than do the anionic surfactants. The polyethoxylated alkyl phenol nonionic surfactants usually exhibit a reverse solubility relationship with temperature and become insoluble at temperatures of above their cloud points making them ineffective in many oil formations. Nonionic surfactants that remain soluble at elevated temperatures are generally not effective in reducing interfacial tension. Other types of nonionic surfactants hydrolyze at temperatures above about 75.degree. C.
The use of certain combinations of anionic and nonionic surfactant to combat hard water formations has also been suggested. For Example, U.S. Pat. No. 3,811,505 discloses the use of alkyl or alkylaryl sulfonates or phosphates and polyethoxylated alkyl phenols. U.S. Pat. N. 3,811,504 discloses the use of three component mixture including an alkyl or alkylaryl sulfonate, an alkyl polyethoxy sulfate and a polyethoxylated alkyl phenol. U.S. Pat. No. 3,811,507 discloses the use of a water-soluble salt of a linear alkyl or alkylaryl sulfonate and a polyethoxylated alkyl sulfate.
Cationic surface-active materials such as quaternary ammonium salts, and derivatives of fatty amines and polyamines, have also been used. However, these compounds have the disadvantage of substantivity or attraction especially towards silicate rock, and they lose their activity by adsorption.
The use of certain amphoteric surfactants which function as cationics in acid media and become anionic when incorporated in alkaline systems has been suggested. For example, U.S. Pat. No. 3,939,911 discloses a surfactant waterflooding process employing a three-component surfactant system. This surfactant system includes an alkyl or alkylaryl sulfonate such as an ammonium dodecyl benzene sulfonate, a phosphate ester sulfonate, and a sulfonated betaine such as a C.sub.12 -C.sub.24 alkylamide C.sub.1 -C.sub.5 alkane dimethylammonium propane sulfonate.
The use of hydrocarbyl-substituted polyoxyalkylene sulfonates is disclosed, for example, in U.S. Pat. Nos. 3,916,994; 4,181,178; 4,231,427; 4,269,271; 4,270,607; 4,296,812; 4,307,782; 4,316,809; 4,485,873; and 4,478,281.
U.S. Pat. Nos. 4,468,335 and 4,468,342 disclose the use of branched alkyl-substituted polyethoxypropane sulfonates. In this regard, the '335 patent, in a preferred embodiment, discloses the use of compounds of the formula EQU RO(C.sub.2 H.sub.4 O).sub.x CH.sub.2 CH.sub.2 CH.sub.2 SO.sub.3 Na
wherein R is ##STR4## in which R.sub.1 has 7-26 carbon atoms, R.sub.2 has 2-10 carbon atoms, and R.sub.3 has 6-26 carbon atoms. The '342 patent discloses the use of blends of at least two homologous surfactants of the formula EQU R(OC.sub.2 H.sub.4).sub.n -OCH.sub.2 CH.sub.2 CH.sub.2 SO.sub.3 Na
wherein R is ##STR5## in which R.sup.1 and R.sup.2 are the same or different alkyl groups of 4-24 carbon atoms and the total number of carbon atoms in R.sup.1 and R.sup.2 is 10-30, and n is 2-6.
The use of thickening agents to increase the viscosity of injected water, normally to a value of at least equal to that of the reservoir oil, in order to arrive at a favorable mobility ratio between the oil and water and increase the macroscopic displacement efficiency of waterflood is known. Examples of such thickeners or mobility control agents are Polysaccharide B-1459 available from Kelco Company under the tradename "Kelzan" or the partially hydrolyzed polyacrylamides available from the Dow Chemical Company under the tradename "Pusher" chemicals.
While many surfactant waterflooding methods have been proposed, there is a substantial, unfulfilled need for surfactants and waterflooding methods employing such surfactants that are useful in recovering oil from subterranean formations wherein the surfactants employed are exposed to high temperatures, high salinities, high pressures, and high concentrations of divalent metal ions.